1. Field of the Invention
The present invention relates to a photoresist composition comprising a resin which enables a deep ultra violet (hereinafter referred to as "DUV") beam to be used as a light source, said photoresists composition having high thermal resistance and etch resistance. More particularly, the present invention relates to the use of polymaleic imide resin in a photoresist composition, thereby improving the production yield and reliability of semiconductor devices. Also, the present invention is concerned with a preparing method of the photoresist composition and a method for forming a fine pattern from the photoresist composition.
2. Description of the Prior Art
Using a pattern formed on a silicon substrate as a mask, dopants are implanted in the silicon substrate under an accurate control, and the implanted regions in the silicon substrate are interconnected to form elements or circuits. The pattern defining such implanted regions is usually obtained by an exposure process. A photoresist film is coated on a wafer and exposed, through a patterned mask, to light with a certain wavelength, for example, uv ray, electron beam or X ray. Thereafter, taking advantage of the solubility difference between the exposed region and the unexposed region, the photoresist film is developed to form a pattern. This pattern functions to protect the substrate, as well as serving as a barrier upon etching or ion-implantation.
A photoresist film composition typically comprises a resin consisting of polymer, a photosensitive acid generator, and other additives. The polymer, a base of the photoresist film, has a structure in which an alkyl group is repeated forming a large chain. Upon exposing the composition to a beam with a certain wavelength, a photochemical reaction occurs via the photosensitive acid generator.
The photoresist pattern prepared from the composition has a resolution power represented by the following formula: EQU R=k.times..lambda./NA
wherein k is a process constant; .lambda. is a wavelength of a light source; and NA is a numerical aperture. As indicated, the resolution power is proportional to the wavelength and the process constant but inversely proportional to the numerical aperture. It is very difficult to reduce the process constant and increase the numerical aperture to certain values. This is the main factor in improving the resolution. In fact, it is virtually impossible to form a fine pattern necessary for 1 giga or higher integration of a semiconductor device by controlling the process constant or numerical aperture.
Accordingly, research has been directed to find new light sources suitable to improve the resolution power. As a result, a DUV ray was developed as a light source for the integration of semiconductor devices into 1 giga or a higher scale. Examples of the DUV light source include krypton fluoride excimer laser (hereinafter referred to as "KrF") and argon fluoride excimer laser (hereinafter referred to as "ArF") which are 248 nm and 193 nm in wavelength, respectively. According to the finding of a new light source, a suitable photoresist film should be developed.
In order to better understand the background of the present invention, a description will be given of a conventional photoresist film resin for DUV rays.
A conventional photoresist film composition for KrF comprises a polyvinylphenol resin and a photosensitive acid generator as follows: ##STR1## wherein R is an alkyl moiety functioning as a dissolution inhibitor. A typical dissolution inhibitor is the tert-butoxycarbonyloxy group.
When the photosensitive acid generator is exposed to UV, a proton is generated and, then, reacts with the alkyl moiety to generate another proton as follows: ##STR2## When the alkyl moiety is tert-butoxycarbonyloxy, R.sub.1 is isobutylene. In sequence, the proton is used for such a reaction. Thus, the resin containing no dissolution inhibitor moiety is dissolved in a developing solution while the unexposed resin is not dissolved.
The polyvinylphenol type resins used for KrF light source show maximal uv absorbance around 210 nm and 270 nm. When a pattern is formed away from the absorbance range using KrF (248 nm) as a light source, the polyvinylphenol type resins can be used as photoresist resins. However, in a lithography process using ArF (193 nm) as a light source, any resin that absorbs ArF light, including polyvinyl type resin, cannot be used as a photoresist resin. The reason is that the resin showing a maximal absorbance at 210 nm can also absorb the beam with a wavelength of 193 nm. Owing to this, the photochemical reaction of the photosensitive acid generator is restrained. The resin, if used, gives an altered pattern.
Currently, a polymethylmethacrylate (hereinafter referred to as "PMMA") resin, represented by the following structural formula I, or an aromatic resin, represented by the following structural formula II, is used as a photoresist resin for ArF: ##STR3## R=dissolution inhibitor wherein R is an alkyl moiety functioning as a dissolution inhibitor.
The photoresist film patterns from these resins, however, are so poor in etch resistance and thermal resistance that they are unsuitable for forming highly fine patterns of high density. Further, the photoresist film patterns are difficult to use in practice because they are altered.